How
does an hourglass measure time?

By
using carefully calculated amounts of sand.

The hourglass is sometimes referred to as a sand clock or a
sandglass. Like other timepieces, it needs to be carefully calibrated.
The
hourglass maker must test the instrument and fine tune it to
measure the correct length of time.

There
are many factors that contribute to the ability of an hourglass
to accurately measure time. The type and quality of sand is key.
It must have a rate of flow that does not fluctuate. Sand that
is too coarse will wear away the glass, eventually making the
neck too large. Most important is the ratio of the neck (the hole,
or tube) width to the diameter of the sand particles.

Here
are the other factors that affect the accuracy of an hourglass:

The
amount or volume of sand used

The
size and angle of the glass bulbs

The
quality of the sand or granular material. It must be fine, dry
and consistently formed so it can flow smoothly. (Some substances
used in the past were fine grain sand, powdered eggshells, and
powdered marble.)

The
width of the neck

A
tight seal so no moisture can get into the chambers. Moisture
can add weight to the sand or clog up the neck.

A
flat and level surface on which to rest the hourglass

We still use
the hourglass to keep track of time. Just think of its many uses
for cooking and for playing games!

Turner,
A.J. The accomplishment of many Years: Three notes towards
a history of the sand glass. In Of time and measurement:
studies in the history of horology and fine technology
by A.J. Turner, Brookfield, VT, Variorum, c1993. p. 161-172

A
poster comes to life. The same kind of tiny hourglasses which
time the nation's three-minute breakfast eggs are used to measure
the heating time of steel in an annealing oven, where the metal
is "cooked" at temperatures up to 1,400 degrees Fahrenheit.
Allegheny-Steel, Pittsburgh, Pennsylvania. Alfred T. Palmer, photographer,
1942. Prints and Photographs
Division, Library of Congress.